1. Field of the Invention
The present invention relates to an optical disk, a recording/reproducing method of the optical disk, and an optical disk apparatus with employment of the optical disk.
2. Description of the Related Art
Very recently, various sorts of technical ideas capable of recording on recording media in a multi-layer manner have been proposed in order to increase recording capacities of the recording media.
In other words, in the recording media, micro-hologram layers constructed of plural layers are formed in inner portions of plate bodies along thickness directions of the plate bodies. Since the micro-hologram layers are made in the multi-layer structures, recording capacities of the recording media become extremely large (refer to, for instance, below-mentioned publication 1).
Patent Publication 1: U.S. Pat. No. 7,388,695
In the above-described conventional techniques, an NA (numerical aperture) of a lens employed so as to form the micro-hologram layers made of the plural layers in the inner portions of the plate bodies is made small in order that light may reach the inner portions of the plate bodies.
Then, while this lens is employed, micro-holograms having the same dimensions are formed in a micro-hologram layer provided on the side of a plate body, and furthermore, in another micro-hologram layer provided on the side of an inner layer of this plate body.
However, as previously explained, as to the micro-holograms formed in the respective micro-hologram layers by employing the lens having such a small NA, dimensions of these micro-holograms along a thickness direction of the plate body become longer. As a result, a large number of the above-described micro-hologram layers cannot be formed within the plate body, which may impede that recording capacities thereof are increased.